1. Field of the Invention
The subject invention relates to bioelectrodes applicable to body parts of living subjects to detect electrical signals and for similar purposes.
2. Description of the Prior Art
Bioelectrodes have sometimes been designed so as to permit direct physical contact of an electrode or a metallic part acting as an electrode with the skin of the subject to which the electrode is applied. Such direct physical contact has prevented proper operation of the electrode in its requisite electrolytic environment and has, moreover, introduced artifactitious signals or motion generated noise into the detected electrical body signals, thereby rendering the bioelectrode inoperative for its intended purpose.
In order to overcome this disadvantage, the prior art evolved electrode designs in which the metallic electrode was maintained spaced from the skin of the subject and in which a liquid or jellylike electrolyte is disposed between the electrode and the subject's skin. In practice, that design did not generally perform well because of the fluid nature of the electrolyte which did not assure a constant and reliable electrolytic contact between the electrode and the subject's skin.
To overcome these disadvantages, it has been proposed to cover the electrode with an electrolyte-absorbent pad, such as a piece of sponge or felt, for reliably retaining the requisite electrolyte between the electrode and the subject's skin. In order to provide for an intimate contact of the electrolyte pad with the subject's skin and in an effort to reduce the loss of electrolyte from the pad during the use of the electrode, a circular skirt of rubber or another material having a flexibility similar to that of the skin has frequently been arranged around the electrolyte pad. In practice, this encumbered the assembly of the electrode structure and introduced problems of wear and tear. Moreover, this "suction cup" approach did not consistently assure the requisite uniform application of the electrode pad to the subject's skin.
Accordingly, a type of design evolved in which the bioelectrode included a cup of electrically insulating material or an equivalent structure that defined a cavity at the bottom of which the metallic electrode was located. That cavity was provided with an electrolyte pad which essentially completely filled the cavity and tended to project to the outside thereof for an intimate contact with the subject's skin.
The feature of the electrolyte pad essentially completely filling the electrode structure cavity was thought important for several reasons.
For instance, some researchers strove for a maximum cross-section of the electrolytic current conducting paths by filling the entire cross-section of the electrode structure cavity with electrolyte retaining material.
Other prior-art workers essentially completely filled the electrode cavity with an electrolyte retaining pad in order to preclude the existence of any substantial clearance between the electrolyte pad and the inner walls of the cavity. There were several reasons for this prior-art methodology. In some instances, an intimate abutment of the electrolyte pad against the lateral inside walls as well as against the bottom of the cavity was thought important for the achievement of an intimate pressure of the electrolyte pad against the subject's skin. In other instances, the existence of free space between the electrolyte pad and the cavity walls was thought to foster an accumulation of extruded electrolyte which could have interfered with the sealing of the electrode structure in cases where a protective stripable cover was applied to the electrode pad and was retained on the electrode cavity by an adhesive.
Another reason for essentially completely filling the cavity with the electrolyte pad was added to the above mentioned list when open-cell foamed pads of urethane or another material which is not of itself electrolyte-absorbent came into use as electrolyte retaining vehicles in bioelectrodes.
Typically, such open-cell pads are filled with electrolyte during the manufacture of the bioelectrodes. Equally typically, these types of electrolyte pads are provided with a thickness greater than the depth of the electrode structure cavity whereby the pad tends to project outwardly of the cavity into intimate contact with the subject's skin. To appreciate the difficulty prevailing with that type of prior-art bioelectrode, it is necessary to visualize that in the case of an open-cell pad of a material that is hydrophobic and thus not of itself capable of retaining any electrolyte, all the electrolyte stored in the pad is of necessity contained inside the open cells thereof. Accordingly, electrolyte is inevitably extruded from the pad when the cells thereof are diminished in size. The latter is the case when the electrolyte pad is compressed into the cavity.
No serious consequences would ordinarily flow from such an extrusion of electrolyte if the pad compression took place when the bioelectrode is applied to the subject's skin. However, if the protruding electrolyte pad is depressed during the manufacture of the bioelectrode, such as during the application of a removable cover to the cavity, and is then relaxed prior to the application of the bioelectrode to the subject's skin, such as at the occasion of a removal of the latter cover, serious consequences can result from the existence of a free space between the electrode pad and the wall of the electrode cavity filled with air.
The reason for this resides in the fact that electrolyte is extruded into that free space when the electrolyte pad is first compressed, such as during the application of a protective cover as indicated above. When thereafter the electrode pad is relaxed, such as during the removal of the protective cover prior to an application of the electrode to the skin, there is no guarantee that the extruded electrolyte will be completely sucked back into the pad. Rather, most of the extruded electrolyte will not be able to reenter the cells inside the pad in a typical situation but will be replaced by air. Accordingly, since the pad in the prior-art version under consideration is not of an electrolyte-absorbent material, the electrolyte paths inside the pad will be interrupted by the entrapped air when the pad is decompressed or relaxed prior to application of the bioelectrode to the subject's skin.
Due to the persistence of these difficulties, the prior art eventually evolved a design in which the traditional feature of the electrolyte pad essentially completely filling the electrode cavity was retained and in which a removable protective cover had a raised portion overlying the pad and having a height greater than the height of the cavity so that the applied protective cover would accommodate the greater thickness of the pad above the electrode cavity. This, however, generated some further problems so that it eventually became necessary in practical bioelectrode devices to provide the raised portion of the protective cover with a projection which engaged the electrolyte pad for holding that pad inside the electrode cavity.
This, in turn tended to deplete a central region of the pad of electrolyte and to impose a permenant deformation on the pad during prolonged storage, thereby placing into jeopardy the desired uniform contact of the electrolyte with the subject's skin. Moreover, the increased height of the assembled bioelectrode necessitated by the above mentioned raised cover portion worked against the modern trend toward low-profile bioelectrodes and toward increased savings of materials which are periodically in short supply.
In a similar vein a type of prior-art electrode provided an electrode plate with lateral hooks which were driven into the side of the electrolyte sponge. This was highly undesirable from an electrochemical point of view, since these hooks encouraged formation of motion sensitive, artifactitious current conducting paths along the side of the electrolyte pad and in detrimental shunt relationship to the path through and inside the pad between the main proper of the electrode and the adjacent body part. Also, these hooks impaired the structural integrity of the electrolyte pad and required special equipment and steps for their application to the pad.
Due to the persistence of these difficulties, the prior art eventually evolved designs in which the electrolyte pad was simply placed into the electrode cavity and was retained therein by a removable protective cover.
This, of course, engendered the danger that the electrolyte pad would fall out of the cavity when the cover was removed prior to application of the electrode to a body part. The prior art, therefore, provided the removable cover with a projection which engaged the electrolyte pad for holding that pad inside the electrode cavity. However, that proposal could not operate satisfactorily, since that projecting portion depleted a central region of the pad of electrolyte and imposed a permanent deformation on the pad during prolonged storage, thereby placing into jeopardy the desired uniform contact of the electrolyte with the subject's skin. Moreover, the electrolyte pad still tended to fall out of the electrode cavity when the cover with the mentioned projection was removed from the electrode proper.